Blueward radial velocity of a neutral (blue line) or ionized (red line) magnesium atom escaped from HD 209458b, as a function of the distance from the planet center. Diamond symbols show the time since the particle escape with a step of one hour (half-hours are indicated by cross symbols). A neutral particle reaches 60 km s^-1 (horizontal dotted line) in about 4.25 h at 4 R_p and an ionized particle in about 5.2 h at 8 R_p.